In marine biology: a male fish that disguises its appearance as a female to sneak into other males nests to fertilize eggs.

In human biology: That somewhat effeminate guy who is "just a friend" of your girlfriend, but will take a stab at her if you get in an argument or are travelling.

Guy: That Aaron guy drives me nuts - do we have to hang out with him?
Girl: Oh, Aaron? He's just a friend! Don't worry!
Aaron (in background, to self): "I'm gonna pound your girls ass the first chance I get!" Aloud: "Oh girl, do you want to go shoe shopping?! I heard there's a sale on and afterward we can chat over coffee!"
Guy (to self): Damn sneaker male!